A diffraction grating is utilized in optical systems of various devices, as a spectral element having a periodic structure composed of a large number of parallel members, and, in recent years, its application to X-ray imaging devices has also been attempted. In terms of a diffraction process, the diffraction grating can be classified into a transmissive diffraction grating and a reflective diffraction grating. Further, the transmissive diffraction grating includes an amplitude-type diffraction grating (absorptive diffraction grating) in which a plurality of light-absorbing (absorptive) members are periodically arranged on a light-transmissive substrate, and a phase-type diffraction grating in which a plurality of optical phase-shifting members are periodically arranged on a light-transmissive substrate. As used herein, the term “absorption (absorptive)” means that light is absorbed by a diffraction grating at a rate of greater than 50%, and the term “transmission (transmissive)” means that light is transmitted through a diffraction grating at a rate of greater than 50%.
A diffraction grating for near infrared light, visible light, or ultraviolet light can be relatively easily produced, because near infrared light, visible light and ultraviolet light are sufficiently absorbed even by a thin metal. For example, an amplitude-type diffraction grating can be produced by subjecting a metal film formed on a substrate such as glass by vapor deposition to patterning to form a grating structure. In an amplitude-type diffraction grating for visible light, when aluminum (Al) is used as a metal, it is enough for the metal film to have a thickness, for example, of about 100 nm, because a transmittance of aluminum with respect to visible light, i.e., a transmittance of aluminum with respect to electromagnetic wave having a wavelength of about 400 nm to about 800 nm, is 0.001% or less.
On the other hand, as is well known, X-ray is very low in terms of absorption by a material, and is not so large in terms of phase shift, in general. Even in the case where an X-ray absorptive diffraction grating is produced using gold (Au) as a relatively favorable material, a required thickness of gold is about several ten μm or more. As above, in an X-ray diffraction grating, when a periodic structure is formed by arranging a transmissive member and an absorptive member or phase-shifting member which are even in width, at a pitch of several μm to several ten μm, a ratio of thickness to width (aspect ratio=thickness/width) in the gold portion has a high value of 5 or more.
Meanwhile, when a plurality of individual members constituting a periodic structure lie parallel to each other, X-rays enter a peripheral region of a diffraction grating obliquely, as depicted in FIG. 17A, because the diffraction grating has a high aspect ratio, as mentioned above, and an X-ray source for radiating X-rays is generally a spot wave source. Consequently, X-rays are not transmitted through the diffraction gating in the peripheral region, thereby leading to the occurrence of so-called “vignetting”. As means to suppress the occurrence of vignetting, there is an idea of forming the members of the periodic structure to extend along respective light rays radiated from the spot wave source. Specifically, for example, it is conceivable to form a diffraction grating in a curved shape, as depicted in FIG. 17B.
Examples of a manufacturing method for a diffraction grating having such a curved periodic structure include a microstructural body manufacturing method described in the following Patent Literature 1. This microstructural body manufacturing method is a method for manufacturing a microstructural body comprising a mold which has a microstructure and a plated layer in an obverse side thereof and has a curved surface in a reverse side thereof. The method comprises the steps of: providing a mold having a microstructure formed by depthwise etching using anisotropic etching, wherein the mold is imparted with electrical conductivity at a bottom of a continuous gap in the microstructure; performing plating from the side of the bottom in the microstructure to form a first plated layer within the continuous gap in the microstructure; and forming a second plated layer capable of generating stress to cause the mold to become curved due to the stress arising from the second plated layer.
Meanwhile, in the microstructural body manufacturing method disclosed in the Patent Literature 1, when the mold is curved by the stress arising from the second plated layer, an excessively large thickness of the mold can cause an insufficient curvature. On the other hand, if a thin mold easy to be curved is employed, or the mold is thinned by polishing or the like before forming the second plated layer, in order to obtain a sufficient curvature, a problem such as breaking (crack) of the mold during manufacturing is more likely to occur. Further, in a situation where the mold is insufficiently curved due to an excessively large thickness thereof, it is conceivable to thin (reduce a thickness of) the insufficiently-curved mold by polishing to thereby allow the mold to become largely curved. However, this is not realistic because of difficulty in realizing desirable polishing. In such a manufacturing method using a stress layer such as the second plated layer, there is difficulty in manufacturing a grating structure largely (steeply) curved with a relatively small curvature radius, or a problem during manufacturing, such as crack of a grating structure during manufacturing, is more likely to occur. Therefore, there is a need for a manufacturing method capable of manufacturing a grating structure steeply curved with a relatively small curvature radius, while suppressing the occurrence of a problem during manufacturing of such a steeply curved grating structure, i.e., ensuring sufficiently high handleability.